An exhaust emission control device having a selective reduction type NOx catalyst (hereinafter, referred to simply as an “NOx catalyst”) to remove NOx contained in an exhaust gas discharged from an internal combustion engine by using ammonia as a reducing agent is known. A supply valve or the like that supplies ammonia or a precursor of ammonia (for example, urea) into an exhaust gas upstream of the NOx catalyst is installed in the exhaust emission control device so that an adsorption amount of ammonia by the NOx catalyst is appropriate for NOx removal.
Even if the NOx catalyst is normal, however, the amount of ammonia that can be adsorbed by the NOx catalyst decreases due to a temperature rise of the NOx catalyst so that ammonia may flow out from the NOx catalyst. On the other hand, purifying ammonia flowing out from the NOx catalyst by arranging an oxidation catalyst downstream of the NOx catalyst is known. Then, a technology to determine abnormal conditions of the NOx catalyst or the oxidation catalyst based on a value of an NOx sensor downstream of the oxidation catalyst when the oxidation catalyst is arranged downstream of the NOx catalyst and the amount of urea supplied to the NOx catalyst is increased is known (see, for example, Patent Literature 1). Also, the conversion of ammonia to NO in an ammonia slip catalyst provided downstream of the NOx catalyst is known (see, for example, Patent Literature 2). In addition, the conversion of ammonia to NO in the ammonia slip catalyst provided downstream of the NOx catalyst at 400° C. or higher is known (see, for example, Patent Literature 3). Further, it is known that, when the conversion of ammonia to NO occurs in a sensor cell of the NOx sensor, the sensor output increases (see, for example, Patent Literature 4).
When the temperature of the NOx catalyst rises, ammonia may be released from the NOx catalyst. Such a release of ammonia may occur also in a normal NOx catalyst and the release itself is not an abnormal condition of the NOx catalyst. Also, depending on the temperature of the NOx catalyst, the temperature of the oxidation catalyst, and the flow rate of exhaust gas, ammonia flowing out from the NOx catalyst may not be oxidized by the oxidation catalyst or ammonia flowing out from the NOx catalyst may be converted to NOx even if the oxidation catalyst is normal. Therefore, even if the NOx catalyst and the oxidation catalyst are normal, ammonia may flow out downstream of the oxidation catalyst or NOx converted from ammonia may flow out.
Incidentally, the NOx sensor also detects ammonia and when the NOx sensor is installed downstream of the oxidation catalyst, NOx and ammonia flowing out from the oxidation catalyst are detected. Thus, if the NOx removal rate is calculated based on the NOx concentration upstream of the NOx catalyst and the NOx concentration downstream of the oxidation catalyst, the NOx removal rate is calculated lower due to ammonia and NOx flowing out from the oxidation catalyst with a rising temperature of the NOx catalyst. Then, if the deterioration of an exhaust emission control device is determined based on the NOx removal rate, there is the possibility that the exhaust emission control device is determined to have deteriorated even if the NOx catalyst and the oxidation catalyst are normal.